A hybrid electric automobile, an electric automobile, and the like have a constitution in which a motor is driven by an electric power from a DC power supply such as a secondary battery. In this case, the constitution in which a DC motor is directly driven by the electric power from the DC power supply is rarely employed. Instead, the electric power from the DC power supply is supplied to an inverter, and an AC (alternate current) power or a DC power to be supplied to the motor is generated by the inverter. By using the inverter, the control of a rotational frequency and an output torque of the motor can be carried out by a switching control at the inverter. In a case of a hybrid electric automobile, a battery pack of lithium-ion secondary battery cells, for example, is used for the DC power supply. A terminal voltage of the battery pack is 288 V, for example.
In a case of a usage for the hybrid electric automobile and the electric automobile, since a DC power supply with the voltage of 200 V or higher which is capable of making a large current flow is used, the relay contact is inserted, for the purpose of security or the like, in the power supply line on each of a positive side and a negative side of the DC power supply; and thereby the DC power supply is completely disconnected from a side of the load circuit such as the inverter when the DC power supply is not used. In addition, in a case of the usage for driving a motor, there occurs a considerable fluctuation of the load and an input voltage to the load circuit such as the inverter remarkably fluctuates in accordance therewith; and accordingly, in order to alleviate the fluctuation, a smoothing capacitor of a large capacity is provided between the positive side and the negative side of the power supply line on an input side of the load circuit.
FIG. 1 shows one example of the circuit which uses the battery pack constituted by serially connecting a plurality of cells of the secondary battery as the DC power supply and in which the relay contact is inserted for each of the positive power supply line and the negative power supply line from the DC power supply.
The circuit supplies the electric power from battery pack 10 to load circuit 16 such as the inverter circuit. A contact of first main relay 11 is inserted in the power supply line extended from the positive pole of battery pack 10 to one end of load circuit 16. Besides, an electric connection between battery pack 10 and load circuit 16 can be controlled by an ON (a conductive state)/OFF (an interruption state) control of first main relay 11. Similarly, a contact of second main relay 12 is inserted in the power supply line extended from the negative pole of battery pack 10 to the other end of load circuit 16. Capacitors 15 with a large capacity is provided in parallel to load circuit 16. Because of the provision of capacitor 15 here, it is possible that the contacts of main relays 11, 12 are fixed or welded due to a large rush current flowing into capacitor 15 when the contacts of main relays 11, 12 are turned from the interruption state to the conductive state. If the welding like this occurs, the relay contact does not transit to the interruption state again, and the function as the relay is lost. Of course, depending on the electrical characteristics of load circuit 16 itself, even in a case where there is no capacitor 15 specially provided, the large rush current large enough to weld the relay contact may sometimes flow in the circuit.
In this regard, a series circuit comprising resistor 14 and a contact of precharge relay 13 is provided in parallel to the contact of main relay 11 to prevent the welding of the relay contact. In other words, in a case where the electric power is supplied from the interruption state to load circuit 16, first, second main relay 12 on the negative side is set to be ON state; and next, the precharge relay 13 is set to be ON state. As a result, a charging current to capacitor 15 flows via resistor 14, and capacitor 15 is gradually charged. After that, first main relay 11 on the positive side is set to be ON state, then, following this, precharge relay 13 is set to be OFF state; and thereby the electric power is supplied from battery pack 10 to load circuit 16 via first main relay 11 and second main relay 12 without generating the large rush current.
However, if the precharge relay is provided, still it is possible that the welding of the contact of the main relay occurs. This is one of the reason why the main relays are provided for both of the positive and negative sides. In this regard, it is necessary to detect the occurrence of the welding at the contact of the main relay.
Japanese Laid-Open Patent Application No. 2000-270561 (JP, P2000-270561A) discloses that in a case where an inverter circuit is used as the load circuit, the welding of the main relay is determined by determining whether or not the output current of the inverter converges to zero after a lapse of a prescribed period of time when both of the main relays on the positive side and the negative side are controlled to be off. If the output current of the inverter does not converge to zero after the lapse of the prescribed period of time, this means that the electric power is supplied from the DC power supply side even when the main relay is controlled to be off. Accordingly, it is determined that the welding of the contact of the main relay occurs. However, with this method, it cannot be determined that the welding occurs unless the welding occurs at the contacts of both main relays and also it cannot be detected that the welding occurs at only one of either of the main relays.
Japanese Laid-Open Patent Application No. H06-233401 (JP, 6-233401, A) discloses that the relay having a mechanism for detecting the state of the movable part thereof is used as the main relay; and an abnormality such as welding and the like is detected when the movable part of the relay is at a position corresponding to the conductive state even when the relay is not controlled to be in the conductive state. However, in this method, it is necessary to provide the main relay with a special mechanism for detecting the state of the movable part thereof; and therefore this method lacks in versatility and also it is necessary to sufficiently pay attention to a reliability of the special mechanism itself.
Japanese Laid-Open Patent Application No. 2000-78851 (JP, P2000-78851A) discloses that in a case where the relay for compulsorily discharging the capacitor, namely a discharge relay, is provided in the circuit similar to the circuit as shown in FIG. 1, the resistor is serially inserted for the contact of the discharge relay in order to prevent from the contact of the discharge relay from being welded. However, the method in which the resistor is serially inserted in relation to the relay contact cannot be applied in reality for the main relay as mentioned above in which the contact is directly inserted between the DC power supply and the load circuit.
Japanese Laid-Open Patent Application No. 2000-173428 (JP, P2000-173428A) describes a constitution such that a first welding detection circuit using a photo-coupler including a light emitting device connected between the DC power supply side of the contact of one main relay and the load circuit side of the other main relay; and a second welding detection circuit using a photo-coupler including a light emitting device connected between the DC power supply side of the contact of the other main relay and the load circuit side of one main relay, in the same way; are provided; and wherein the welding at the contact of the main relay can be independently detected by using these welding detection circuits. However, in this constitution, it is necessary to provide as many as two systems of the welding detection circuits which are directly connected to the DC power supply of 200 V or higher; and this results in requiring a high cost for the whole apparatus.
Under the present circumstances, in the system in which the contact of the main relay is inserted in each of the power supply line on the positive side from the DC power supply and the power supply line on the negative side from the DC power supply, when the welding of at least one of the main relay contacts occurs, there is no method for independently detecting whether the welding occurs on the positive side or the negative side without using an additional detection circuit and the special relay.